1. Field of the Invention
The present invention is directed generally to tank inventory control systems and more particularly to inventory control systems using ultrasonic measuring devices.
2. Description of the Prior Art
Whenever fluids are stored in a tank, the need arises to accurately determine the volume of fluid in the tank. Currently, the normal practice at sites storing Class I liquids (gasoline and diesel), is to measure the amount of fluid or product in the tanks from one to three times per day. The current standard method of assessing tank inventory levels makes use of a wooden stick, known as a "tankstick," which is dipped into the tank to measure the height of the product in the tank, in inches. Obviously, with that method of measurement, the reading, recording, and converting to gallons must all be done manually. That provides too much opportunity for human error. Totalizers (on the gas pumps or an electronic console) are also read manually and the volume dispensed is computed from the previous reading. Those readings are used to provide an inventory reconciliation for the period since the last readings. That short term balance must be stored and later combined with other short term balances to see if a long term trend shows a loss of product through a possible leak. The potential for human error is significant, resulting in inaccurate totals or loss of records. Simple neglect in following the proper procedures is also a common problem.
Shortcomings of the current method are numerous and include such considerations as: the tankstick is cumbersome, usually twelve to sixteen feet long; when the stick is removed, there may be product as high as ten feet up from the bottom of the stick and there is no way the attendant can keep product from getting on his hands; attendants commonly use some powder, like baby powder, on the stick to make it easier to read, which adds impurities to the product; and the attendant must carry the stick, something to write on, something to write with, and still have a free hand to get the cap off the tank fill pipe.
The stick's accuracy is normally plus or minus one-fourth inch. In a typical size tank, that translates to an accuracy of plus or minus fifty gallons. The accuracy of such measurements further suffers from the following problems: dropping the stick into the tank eventually causes a dent in the bottom of the tank, which adds to any inaccuracies; product can splash up on the stick causing false readings; most sticks are wood, and older sticks are porous enough that the product wicks up the stick giving erroneous data; in dry areas, the lighter petroleum products, like gasoline, immediately start to evaporate from the stick making accurate readings impossible; the end wears off the stick with use, making all readings inaccurate; and the stick is stored outside, usually leaning up against the building, causing it to warp.
The need for more accurate measurements of fluids in underground tanks has been recognized in the art. For example, U.S. Pat. No. 3,394,589 discloses an Apparatus For Measuring Liquid Level. The apparatus may be used, for example, in conjunction with measuring a liquid level in a tank. The apparatus utilizes an electro-acoustic transducer that produces a sound wave directed toward the surface of the liquid along a sound wave guide path. A plurality of reflection elements located at fixed distances from the transducer are associated with the guide path. The sound wave progresses through the sound wave guide towards the liquid surface. As the sound wave progresses downwardly towards the liquid surface, it is partially reflected by the reflection elements. Upon reaching the liquid surface, the sound wave is again reflected. The reflected sound waves are detected by the transducer which produces electrical signals in response to the detection of each wave. An oscillator provides a given number of cycles, or pulses, between the reflection time from a pair of reflectors; by counting the number of pulses, a measure of the liquid level between reflectors can be obtained.
U.S. Pat. No. 4,210,969 discloses a sonic ranging system designed to eliminate the errors due to variations in the sound velocity in the medium above the liquid to be detected in a storage tank. An electro-acoustic transducer is provided with a mounting flange which is used for attaching the transducer to an opening at the top of the storage tank. The transducer is provided with an extending L-shaped rod member which is rigidly attached to the transducer housing structure. A small disk which acts as a sound reflecting target is rigidly attached to the rod so that the flat surface of the target is located perpendicular to the axis of the transducer. The position of the target reflecting surface is fixed at a precise distance from the transducer. The function of the reflecting target is to provide a self-calibrating means for automatically correcting for errors in the measurement of the distance from the transducer to the surface of the liquid which would otherwise occur as a result of variations of the velocity of sound in the air space above the liquid.
The directional sound beam generated by the transducer travels along two paths. The sound beam travelling along one path is reflected from the target and is returned to the transducer while, at the same time, the sound beam travelling along the other path is reflected from the liquid surface and is returned to the transducer at a later time. Counters measure the time between the transmission and the reception of each of the signals. The ratio of the two times as measured by those counters is determined and then multiplied by the known distance between the reflector target and the transducer to determine the distance between the transducer and the surface of the liquid.
Another example of an ultrasonic liquid level meter is disclosed in U.S. Pat. No. 4,470,299. That patent discloses an ultrasonic transducer mounted above a storage tank at a fixed distance from the bottom of the tank. A reflector is also placed at a fixed distance from the transducer. Transmitted energy is propagated through the gaseous medium between the transducer and the liquid surface and is directed both toward the reflector surface and the surface of the liquid. The sound waves are reflected from both surfaces and returned to the transducer which now acts as a detector and whose output is coupled to a receiver.
Each transmitted pulse is followed by a reference echo pulse which corresponds to the receipt of the sound wave reflected from the fixed reflector. The reference echo pulse is followed by a main pulse which corresponds to the receipt of the sound wave reflected from the liquid surface. A clock is started coincident with the transmission of a pulse and stopped when the reference echo pulse is received. The number of clock pulses in that period provides a count representing the reference transit time. A similar clock arrangement provides a count representing the transit time to the liquid surface level. The counts are applied to a microprocessor to yield an output count representing the level of the liquid independent of environmental changes.
Such ultrasonic liquid level measurement devices may be used in conjunction with tank inventory and or tank control systems. For example, in U.S. Pat. No 4,487,065 a system using ultrasonic transducers is disclosed which is capable of providing history reports including tank levels, failure rates, size of tank, etc., user logs, daily gauge reports, tank level reports, user information reports, and lease run tickets.
The need for accurate tank measurements has been heightened recently due to environmental considerations. For many years the Environmental Protection Agency (EPA) has been concerned that leakage from underground storage tanks (especially those containing gasoline and diesel fuel) is contaminating the water table in many parts of our country. The problem is a serious one with damage estimates of fifty-five billion dollars over the next thirty years. Permanent damage to our drinking water supply is a matter that the EPA felt it had to address.
On Sept. 23, 1988, the EPA promulgated final regulations that will affect all owners and operators of underground storage tanks containing petroleum products and other regulated substances. With only limited exceptions, the EPA has issued a basic regulatory directive for the approximately two million underground storage systems in operation today. The new rules address every aspect of the life cycle of an underground storage tank system, i.e. design, construction, installation, upgrading of existing systems, operation and maintenance, cleanup, and closure. These technical requirements, which affect both new and existing underground storage tanks, are directed to three different areas, leak detection, corrosion prevention, and spill/overfill prevention.
All existing underground storage tank systems will be required to achieve required levels of leak detection, corrosion protection, and spill/overfill prevention. The EPA has established schedules for achieving those goals based, in part, on the age of the tank as of the effective date of the final regulations. All existing tanks will be required to have leak detection systems or one of the following three alternatives: (i) monthly monitoring; (ii) monthly inventory control and annual tank tightness testing; or (iii) monthly inventory control and tank tightness testing every five years for tanks with corrosion protection and spill/overfill prevention.
Thus, in view of the need for accurate tank measurement systems, which need has been heightened by EPA regulations, the need exists for a tank level measurement system for providing accurate measurements of fluid levels in tanks. The need also exists for a system for automatically calculating tank volumes based on the measured liquid level. The need also exists for a system capable of performing inventory control functions which can be used for various purposes such as managing inventory or satisfying EPA requirements. The need exists for a system which is inexpensive and which can be installed in existing tanks. The system should be flexible so that it can be used with various tank configurations and should not suffer from the shortcomings found in the prior art.